ITC Stone exists in both Sans and Serif options, and Lucida Sans is paired with Lucida Bright (serif) as well as Lucida Fax (almost slab serif). Adobe's Penumb

ITC Stone exists in both Sans and Serif options, and Lucida Sans is paired with Lucida Bright (serif) as well as Lucida Fax (almost slab serif). Adobe's Penumbra font has a sans--serif axis, well, had. Similarly there was the font Walker which had "snap-on" serifs: https://www.moma.org/collection/works/139311 https://www.moma.org/collection/works/139311 Yes, the distinction is quite subtle for the variable axis options.